Color cathode ray tubes comprise a tube envelope with red, green, and blue phosphor dots on the screen, which emit the chosen color when impinged with a scanning electron beam within the tube. In order to ensure that the correct color dot is energized by the correct applied electron beam, the inside surface of the front of the cathode ray is covered with a shadow mask, (or other similar apparatus, such as an aperture grill plate), which is a metal plate with tiny openings, the metal plate being placed such that the openings are lined up with the color dots on the face of the tube. The shadow mask allows for accurate color reproduction and eliminates color scanning errors. However, the shadow mask plate is metallic and can be magnetized by the earth's magnetic field enough to cause errors to occur when the tube is moved, as when pivoted on a stand on a viewer's desk, for example.
Prior art techniques used to overcome the effects of the earth's magnetic field, or other stray magnetic fields, and improve the color purity of the CRT include using a degaussing coil around the cathode ray tube. When the degaussing coil is energized by a circuit within the device using the CRT, such as a computer, video monitor, or a television set, for example, a compensating magnetic field is produced which nullifies or otherwise negates the effect of the earth's magnetic field or other stray magnetic fields on the shadow mask. Some systems must be manually energized, and other systems energize the degaussing system periodically or upon start-up or initial turning on of the system in which the cathode ray tube is used.
Certain known prior art degaussing techniques are disclosed in U.S. Pat. No. 4,908,550, issued Mar. 13, 1990 to Elaine Chase. This patent discusses the need for degaussing coils, as well as suggesting certain degaussing solutions, such as varying the position of the deflection coil yoke along the CRT neck. Techniques disclosed in the Chase patent includes the use of two coils, near the CRT yoke, and another adjacent to the front of the CRT, but not adjacent the bezel, however.
According to the present invention, a spiral pattern of electrical traces is placed directly on to a plastic part or substrate attached to or adjacent the front bezel that surrounds the CRT. These traces would replace the prior art degaussing coils presently used to eliminate the effects of magnetic fields on the aperture grill or shadow mask of the CRT. The present invention allows the bezel borders on a color CRT display to be very narrow in comparison to the bezel normally needed for prior art CRTs using the bulky, unwieldy, and expensive degaussing cables that are difficult to install. Manufacturability is improved because the degaussing coil can be attached to the bezel, and to the rest of the monitor, with a one axis motion. A second embodiment includes plating the electrical traces directly on to the bezel itself.